Thanks for the feedback!

The tread is held on by pop rivets through holes drilled into the wheel. The tread in question is roughtop from mcmaster, the blue thick kind. It's never done us wrong!

The 4 holes are for holding a sprocket to the wheel.

[edit] This is the wheel for the Coaxial swerve drive that is currently under design. If anyone feels interested in that, I'd be glad to spam our ideas/pictures/CAD files at you over aim. [/edit]